Partition Behavior of Perfluorooctane Sulfonate (PFOS)
and Perfluorooctanoic acid (PFOA) in Riverine Sediments
R.X. Liu*1, J.Y. Tian1,2, B.Li1, X.X.Gong2 and Y.Y.Liu2
1
State Key Laboratory of Environmental Criteria and Risk Assessment (SKLECRA), Chinese
Research Academy of Environmental Sciences, Beijing 100012, China (Email: [email protected])
2
College of Chemical Engineering, Shandong University of Technology, Zibo 255049, China
Abstract
The sorption of perfluoroalkyl substances (PFASs) on the sediments is an important process which
contributes to their fate, distribution and transport in water system. In this study, the sorption
kinetic, thermodynamics and effect parameters on partition behavior of perfluoroctane sulfonate
(PFOS) and perfluorooctanoic acid (PFOA) at low initial concentration were investigated. The
results showed that the equilibrium time was achieved within 48h. The sorption isotherms of
PFOS and PFOA could be described by Freundlich equation. The surface area and composition
of sediment particles as well as solution pH values and ion strength strongly influenced the
sorption of PFOA and PFOS. Both PFOA and PFOS exhibited higher sorption capacity on the
sediments with high organic matter content. These results indicated that multiple driving forces
such as hydrophobic exclusion, specific chemical interaction and electrostatic attraction
contributed to the nonlinear sorption of PFOA and PFOS on sediments.
Keywords
PFOA; PFOS; sediment; partition properties; natural riverine water
INTRODUCTION
Perfluoroalkyl substances (PFASs) are a new type of persistent organic pollutants (POPs) (Wang &
Shih 2011; Wang et al. 2012). They have attracted global concern due to high bioaccumulation,
extreme persistence and toxicity, as well as wide distribution in the environment. Of the PFASs,
perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), ubiquitously present in
the diverse environments (Beskoski et al. 2013; Du et al. 2014), showed the highest persistence and
accumulation in the aquatic environment. Thus, PFOS, its salts and perfluorooctane sulfonyl
fluoride (PFOS-F) have been added to the Stockholm Convention list of the POPs since 2009
(Wang et al. 2009). PFOA, as a Substance of Very High Concern (SVHC) in candidate list under
REACH regulation (ECHA 2015) has been prohibited by the Environmental Pretection Agency of
the United State (US-EPA) on a PFOA-stewardship program in 2006(US-EPA 2015).
However, owing to more than 50 years of production and worldwide application, PFOS and PFOA
can be released into the natural aquatic environment, causing the global distribution of these
compounds (Zhang et al. 2012). Subsequently, these pollutants have undoubtedly involved the
complex processes such as sorption, degradation and transport, which determine their
environmental fate in aquatic systems (Li et al. 2012). The understanding of these basic
mechanisms is of significance in both regulatory and scientific areas. Sediment, with complex
compositions including minerals, clay, sand and sedimentary organic matter, is an important sink
and reservoir of PFOS and PFOA (Zhao et al. 2014). The sorption of PFOS and PFOA onto the
sediment determines their distribution, transport and transformation processes in the aquatic
environment. The sorption behavior highly depends on the physical and chemical properties of
PFOS and PFOA, as well as the conditions of water chemistry such as pH value, ionic strength (IS),
and property of dissolved organic matter (DOM). Higgins and Luthy (2006) evaluated various
sediment-, solution-, and chemical-specific parameters potentially affecting sorption of PFASs.
1
They found that organic carbon rather than inorganic oxide content in sediment was the dominant
effect parameter on sorption, indicating the importance of hydrophobic interactions. The similar
results were also obtained in other previous studies (Ahrens et al. 2009; Ahrens et al. 2010; Zhao et
al. 2012; Zhao et al. 2014; Milinovic et al. 2015), where they reported that hydrophobic interaction
predominated the sorption behavior of longer-chain PFASs that were found to adsorb more strongly
onto sediment, soil or activated sludge while short-chain PFASs were exclusively in water. In
addition, previous work had also demonstrated that sorption of PFASs on sediment, soil or sludge
increased with increasing solution Ca2+concentration and decreasing pH value, in particular, the
sorption behaviors of PFOS was expected to be strongly affected by solution conditions (Higgins
2006; Zhao et al. 2014; Milinovic et al. 2015). These observations suggested that electrostatic
interactions were not negligible in the sorption of PFASs onto sediment. More evidences for both
hydrophobic and electrostatic interactions involved in the sorption mechanism of PFASs were
found by other researchers using aluminum-rich mineral-humic acid as a simulated sediment and
sewage sludges as adsorbents (Zhang et al. 2013; Wang et al. 2015). However, to reduce the
detection errors most of these studies were performed with high initial concentrations of PFASs
ranging from 1 to 1000 µg/L that were much higher than their concentrations (ng/L) in the natural
aquatic environment (Lein et al. 2008; Dufkováet al. 2012; Zhang et al. 2012; Anumol et al. 2013).
The sorptive behavior of PFASs onto natural aquatic sediment highly depends on many factors.
Particularly with high concentration of PFASs to simulate the natural sediment–water interfacial
processes is inadequate and possibly leads to absurd conclusions (Ahrens et al. 2011). For this
reason, this study examined the sorption behavior of PFOS and PFOA on riverine sediment using a
low initial concentration so as to simulate natural aquatic interfacial process. The specific
objectives of this study were: by batch partitioning experiments, to determine the sorption
coefficient and isotherms of PFOS and PFOA onto the sediment and to examine the effect of pH,
ionic strengths, and particle size and density of sediment on the sorption. These would provide
supplementary data for understanding natural sediment-water interfacial process of pollutants.
MATERIALS AND METHODS
Reagents and chemicals
PFOA and PFOS were purchased from AccuStandard Inc (USA). 13C4-labelled PFOA and PFOS
(Wellington’s laboratory) were used as internal standards. Calcium chloride, sodium azide,
hydrochloric acid and sodium hydroxide with analytical reagent grade were purchased from local
chemical companies in Beijing, China. HLPC-grade methanol and ammonium acetate were
supplied by Fisher Company (USA). Milli-Q ultrapure water was used throughout the experiment.
All standard solutions were prepared in methanol and stored in polypropylene (PP) bottles at 4℃.
Sediment collection and characteristics
Four sediment samples were collected from Daliao River systems, Northeast China. The sampling
sites were chosen to reflect a variety of physicochemical properties potentially influencing
partitioning of PFOA and PFOS. The surface sediments were collected in PP plastic bags using a
grab sampler and transferred to the laboratory. The wet samples were air-dried, ground and sieved
through 2 mm meshes. Total organic carbon (TOC) in sediments were measured using a high
temperature TOC analyser (Dohrmann DC-190) after the removal of inorganic carbon by adding
diluted HCl until acidification reaction was completed. Specific surface area (SSA) was analyzed
by N2 sorption method. The pH value of sediment was measured in a 1:2.5 (w/w) mixture of the
sediment with 0.01mol/L of CaCl2 solution by a pH meter. Total Fe and Mn contents in sediment
were determined by an ICP-OES Thermo Elemental (TJA) Iris Intrepid spectrometer following
digestion of sediment with diluted aqua regia (Li et al. 2012). Sediment particle size fractionation
2
was conducted by wet sieving the sediments through 40, 80, 120, 240 meshes to obtain three size
fractions, i.e. 0.42 ~ 0.20, 0.20 ~ 0.125, 0.125 ~ 0.061 mm.
The physicochemical parameters of
sediments were shown in Table 1.
Table 1 Physicochemical property of the sediments
Parameters
sediment 1
sediment 2
pH value
7.58
7.38
OC (mg/g)
8.09
7.98
SSA (m2/g)
104
66.2
Fe (mg/g)
45.8
26.9
Mn (mg/g)
0.82
0.38
sediment 3
7.93
7.97
52.1
19.7
0.40
sediment 4
7.49
80.4
48.1
24.3
0.34
Batch partitioning experiments
Batch experiments included partitioning kinetics, effect of pH value, ion strength and particle size
fractions, as well as sorption isotherm experiments. The partitioning experiments were conducted
in the 50ml polypropylene copolymer (PPCO) Nalgene centrifuge tubes. Duplicate sets of tubes
containing 1.0 g (dry weight) of sediment and 30 ml of 0.01mol/L CaCl2 and 200 mg/L NaN3
solution were spiked respectively with a certain amount of PFOA or PFOS, and equilibrated on the
Thermostatic shaker at 150 rpm and 25℃. To determine an appropriate equilibration time, the
centrifuge tubes with initial PFOA or PFOS concentration of 50 ng/L were shaken for 0 ~ 72 hours
and periodically removed at selected time intervals of 0, 0.5, 1, 2, 4, 8, 12, 24, 36, 48, 60 and 72
hours. The effects of pH, ion strength and sediment particle size on partition behavior were
investigated by batch sorption equilibration experiment at initial PFOS and PFOA concentrations of
50 ng/L. The pH values of aqueous solution were adjusted in the range of 3 ~ 9 with dilute NaOH
or HCl. The concentrations of CaCl2 were 0.001, 0.01 and 0.1mol/L. The sorption isotherm
experiments were carried out with PFOS or PFOA concentrations ranging from 50 to 500 ng/L and
at equilibration time of 48 hours. After shaking, the tubes were centrifuged at 1600r/min for
15min and the supernatants were transferred to the new PP tubes for the extraction and analysis.
For each sample batch, blank samples were prepared using 30 mL of millipore water and treated in
exactly the same way as the samples. The control experiment in the absence of sediment showed
that the total PFOS or PFOA losses was below 5%, which was neglected in the experiment.
Extraction and determination of PFOA and PFOS
The supernatant was passed through Oasis HLB cartridge (Waters, 500mg, 6mL) for the extraction
of PFOA and PFOS. The cartridges were firstly cleaned by dichloromethane and methanol
respectively to remove residual impurities, subsequently preconditioned by 5 mL each of methanol
and ultrapure water (Dufkováet al. 2012; Gong et al. 2016). After extraction, the cartridges were
rinsed with ultrapure water, evacuated to dry for 30 min and eluted with 3×5ml methanol into 15ml
PP tubes. The elution gathered was dried by Nitrogen blowing, and then 1mL of methanol-H2O
(v:v, 1:3) and 10µL of 150 μg/L internal standard solution were added for UPLC-MS-MS analysis.
The concentrations of PFOS or PFOA were determined by UPLC-Xevo TQD (Waters, USA). An
Acquity UPLC® BEH C18 column (2.1×50 mm, 1.7μm) was used for the separation of analyte.
Tandem mass spectrometry was conducted on triple quadrupole mass spectrometer (Xevo TQD,
Waters, USA) equipped with an ESI source. The mobile phases were 2 mmol/L of NH4OAC in
water (A) and in methanol (B). The detailed analytical parameters of the target compounds were
described in the previous study (Gong et al. 2016).
3
Sorption data fitting and partition coefficient
Sorption isotherms were constructed by plotting Csed vs. Cw for PFOA or PFOS-sediment batch
sorption system. The Freundlich model was used to fit sorption data, as described by equations (1)
and (2):
Csed = Kf (Cw)1/n
(1)
or
log Csed = 1/nlogCw + logKf
(2)
where Csed and Cw are concentrations of PFOA or PFOS in sediment (ng/g) and the solution (ng/L)
at equilibrium time; Kf is a Freundlich constant representing the sorption capacity and n is the
Freundlich exponent depicting the nonlinearity of sorption.
The interaction of PFOA or PFOS between sediment and water can be described by its partition
coefficient (Kd):
Kd = Csed/Cw
(3)
By combination of equations (1) and (3), the Kd values were calculated using the equation (4),
which were equilibrium concentration dependent:
Kd = Kf (Cw)(1-n)/n
(4)
Previous studies have shown that the fraction of organic carbon (foc) has an significant effect on the
partition of PFOA or PFOS onto sediment (Higgins and Luthy 2006). Thus, the organic carbon
normalised partition coefficient (Koc) was calculated by the following equation (5):
Koc = Kd/foc
(5)
RESULTS AND DISCUSSION
Sorption kinetics
Figure 1 showed the sorption kinetics of PFOA and PFOS onto sediment. It was obvious that the
sorption process could be divided into three apparent stages: a rapid sorption process, a slow
sorption process and sorption equilibrium process. The rapid sorption occurred at 0 ~ 10 hours for
PFOA and at 0 ~ 5 hours for PFOS. Between10 (or 5) and 40 hours, the sorption rate for PFOA or
PFOS gradually reduced, and then equilibrium was achieved after approximately 40 hours for both
chemicals.
0
10
1.0
CW/C0
0.8
0.6
PFOA
PFOS
0.4
0.2
0
10
20
30
40
50
60
70
80
Time (h)
Figure 1. Sorption dynamic curve of PFOA and PFOS onto the sediment（25℃）
4
This result was slightly different with the previous study where equilibrium time for PFOS and
PFOA was shorter e.g. 4 hours and 8 hours, respectively, when using higher initial concentration
(5µg/ L) of PFOS and PFOA (Arvaniti et al. 2014). This indicated that the initial concentration of
pollutants was one of impact factors controlling their sorptive kinetics between water and sediment.
Simulating natural sediment–water interfacial process of pollutants with higher concentration than
environmental level would lead to unrealistic results. Therefore, at low concentration of PFOS
and PFOA (e.g. 50ng/L) the equilibrium time of 48 hours was adopted for the batch sorption
system.
Effect of solution pH
The effect of pH value on the sorption of PFOA and PFOS was shown in Figure 2. It was
observed that solution pH values in studied range influenced sorption of PFOA and PFOS onto the
sediment to some extent. It was well known that the specific surface species could be formed on
the natural sediment surface. Solution pH value would affect specific pH-dependent reactions on
sediment surface such as ligand exchange, electrostatic interactions and hydrophobic effect (Zhou et
al. 2010; Zhang et al. 2012). Obviously, with increasing pH values between 3 and 7, the sorption
of PFOA and PFOS onto the sediment slightly reduced, which could be explained by the
electrostatic interaction of PFOA and PFOS anionic species with the positively charged sediment
surface at lower pH value. The results were in agreement with Higgins and Luthy’s report
(Higgins and Luthy 2006). It was also noted that a slight increase in sorption of PFOS and PFOA
on the sediment was found at solution pH over 7. Although more negative charges on sediment
surface occurred with pH value increase, resulting in week electrostatic attraction towards PFOA
and PFOS anionic specie, ligand exchange might be predominant on interfacial reactions, leading to
enhanced sorption of PFOS and PFOA on the sediment (Zhao et al. 2014).
100
PFOA
PFOS
Csed / C0 (%)
80
60
40
20
0
2
3
4
5
6
pH
7
8
9
10
Figure 2. Effect of pH value on the sorption of PFOS and PFOA
Effect of ion strength
Figure 3 revealed the sorption trends of PFOS and PFOA on the sediment with varying in Ca2+
concentration from 0.001 to 0.1 mol/L at solution pH value 7. It indicated that the sorption of
PFOS and PFOA increased with increasing Ca2+ concentration, which was in accord with previous
reports by Higgins and Luthy (2006) and by Chen et al (2009). The effect of Ca2+ concentration
on increased sorption percentage of PFOS and PFOA was likely due to several reasons. One was
linked to reduced solubility of PFOS and PFOA as the ionic strength enhanced, thereby increasing
hydrophobic interaction. Chen et al has reported that the sorption-enhancing impact of Ca2+
5
concentration was possibly related to the concentration of PFOS and PFOA in the aqueous phase
with much stronger impact at low PFOS and PFOA concentration (Chen et al. 2009). Furthermore,
with the divalent cation Ca2+ increasing, the electrostatic repulsion between anionic PFOS or PFOA
molecules and the negatively charged sediment surface was reduced because of neutralization of the
negative charges on the sediment surface by the high Ca2+ concentration, which promoted the
sorption of PFOS and PFOA on the sediment. Beyond those Ca-bridging effect initiated by
cations in the solution needed to be taken into consideration (Zhao et al. 2014). It was widely
reported that divalent cations have been found to be able to shift the negative sites of adsorbent
surfaces into positive ones acting as the bridge to electrostatically attract PFASs (Du et al. 2014).
You et al. (2010) also observed that sorption of PFOS on the sediment increased when the CaCl2
concentration increased from 0.005 to 0.5 mol/L, suggesting that Ca-bridging effect was responsible
for this result. Therefore, the formation of Ca-bridging with sediment might influence the sorption
of PFOA and PFOS on the sediment.
100
PFOA
PFOS
Csed / C0 (%)
80
60
40
20
0
0.001
0.01
Concentration of Ca2+ (mol/L)
0.1
Figure 3. Effect of Ca2+ on the sorption of PFOA and PFOS
Effect of particle size
The particle size–dependence of sorption for PFOA and PFOS was shown in Figure 4. The
sorption percentage of target contaminants measured at pH 7 decreased with increasing particle size
of sediment. The results might be attributed to specific surface areas and physicochemical
properties of different particle size. The specific surface areas were decreased with the increased
grain size of sediment while the smaller particles would have larger external surface areas (Yu et al.
2009). The diverse compositions and properties of the fractionated sediment, such as organic
carbon fraction, debris of wood and stalks, mineral grains, might result in different sorption
capacity to PFASs (Zhao et al. 2012).
On the other hand, the compact organic matter was possibly involved with the smallest sediment
particles. It has been reported that the organic matter enhanced the sorption capacity of organic
pollutants onto sediments (Chen et al. 2009; Beskoski et al. 2013; Li et al. 2014). As shown in
Figure 5, the sorption of PFOS and PFOA on sediments was somewhat related to organic matter,
indicating that hydrophobic partitioning played a role in the interaction between target pollutants
and sediment. Therefore, the combination between small-sized sediment and organic carbon was
contributed to the sorption of PFOA and PFOS on sediment, which was consisted with the results
reported by Zhao et al. (2012).
6
100
PFOA
PFOS
Csed / C0 (%)
80
60
40
20
0
0.061~0.125
0.125~0.2
Partical size (mm)
0.2~0.42
Figure 4. Effect of particle size on the sorption of PFOA and PFOS
100
PFOA
PFOS
Csed / C0 (%)
80
60
40
20
0
7.54
8.04
70.24
Content of organic matter (%)
73.43
Figure 5. Effect of organic matter content on sorption of PFOA and PFOS
Sorption isotherms
The sorption isotherms of PFOA and PFOS on sediments were shown in Figure 6. Equilibrium
sorption data was calculated based on the Freundlich model. Over the studied concentrations
range of PFOA and PFOS, all experimental data was fitted very well by the nonlinear Fruendlich
type sorption isotherm with the regression coefficients (R2) of 0.97 for PFOA and 0.95 for PFOS,
respectively. For both PFOA and PFOS, the sorption coefficients (Kf) were 0.0051 and 0.0012,
and the nonlinearity of sorption (1/n) were 0.84 and 0.93, respectively. Regarding nonlinear
sorption isotherms, the partition coefficients Kd and Koc values were function of the solute
concentration, ranged from 0.0012~ 0.0027 and 0.15~0.36 for PFOA, and 0.00061~ 0.00083 and
0.081~ 0.114 for PFOS.
7
This nonlinearity of sorption behavior was mainly attributed to organic and mineral matrices
involved in sediment. Given that hydrophobicity of perfluorinated chain and hydrophilicity of
sulfonate or carboxylate (Zhang et al. 2013), as well as diverse sorption sites on sediments, single
driving force seems to be difficult to explain interfacial reaction between pollutants and sediments.
The hydrophobic exclusion of perfluorinated chain, specific chemical interaction of sulfonate or
carboxylate with sediment, and electrostatic attraction of charged solute molecule with the charged
sediment surface might contribute to the nonlinear sorption of PFOA and PFOS, forming the
sorption of multi-molecular layers.
1.5
PFOA
PFOS
1.0
logCsed
0.5
0.0
-0.5
-1.0
-1.5
1.5
2.0
2.5
3.0
3.5
4.0
logCw
Figure 6. Sorption isotherms of PFOS and PFOA on the sediments
CONCLUSIONS
In natural aquatic system, the sorption of PFOS and PFOA onto the sediments was very low. The
kinetic experiments showed that the equilibrium time was achieved within 48h. The sorption
behavior of PFOA and PFOS could be described by Freundlich equation. Both PFOA and PFOS
displayed higher sorption capacity onto the sediments at high ionic strength and pH values. The
sediment particle with small size and high organic matter content enhanced the sorption of PFOA
and PFOS due to the surface area and specific composition of particles. These results had
important environmental implications: the leaching of PFCs from sediment would become more
serious if background solution was neutral and with low ionic strength. The research would be
benefit for the efficient removal of PFOS and PFOA in the natural aquatic systems.
ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation of China (No.21277133) and
the Basic Scientific and Research Program for Central Nonprofit Research Institutes
(No.2012-YSKY-11)
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